Ans: With increased fin length, the heat transfer area also increases because the heat transfer area is a product of the fin's perimeter and length. Therefore, more and more heat can be transferred from the fin.

But, as the fin length increases, the volume of the fin also increases, and for the same density of fin material the mass of the fin increases.

m = ρ*V (Kg)

With an increase in the mass of the fins, the cost of the fins also increases because the cost of the material is usually $/Kg.

Another fact is as the length and the area of the fin increases the frictional losses also increase. (Drag forces)

Hence, after a certain value, increasing the length of the fin does not justify the added cost of the fin material.

Also, fin efficiency decreases with increasing fin length because of the exponential decrease in fin temperature with respect to length. The temperature of the fin attains to the temperature of the surroundings.

Therefore, designing such an "extra long" fin is out of the question since it results in material waste, excessive weight, and increased size and thus cost with no benefit in return.

Fin lengths that cause the fin efficiency to drop below 60 percent usually can not be justified economically and should be avoided.